A solid state relay provides isolation between a control circuit and a switched circuit and may replace an electromechanical device such as a reed relay. A typical solid state relay consists of a light-emitting diode (LED) optically coupled across an electrically isolating gap to a photovoltaic array. The photodiode array is electrically connected to an output device such as a field effect transistor (FET). Light from the LED creates a voltage across the photovoltaic array and activates the output FET. Alternatively, when light from the LED ceases, the voltage across the photovoltaic array collapses and the output FET is deactivated.
One of the critical limitations of solid state relays is the speed of the switching action. This is the result of a capacitance inherent in the output FET. Each time the photovoltaic array is actuated, this capacitance must be charged before the output FET can turn on. Similarly, each time the photovoltaic array is deactivated, this capacitance must discharge before the output FET can turn off. The charging and discharging of this inherent capacitance inhibits the speed of the switching function. This problem is magnified when high-power circuits must be switched since larger FETs must be used. Accordingly, a larger capacitance must be charged and discharged.
Presently available solid state relays often have relatively slow turn-on and turn-off characteristics and are susceptible to electrical transients. For example, the relay disclosed in U.S. Pat. No. 4,390,790 to Rodriguez includes a photodiode array directly connected to an output FET. Rodriguez's use of a turn-off transistor to discharge the output FET gate-to-source capacitance provides some improvement in turn-off speed, but provides no transient protection to the relay. In a second embodiment disclosed in the Rodriguez patent, an optically coupled JFET switch is used to couple the switched voltage to the gate of the output FET. In so doing, the output FET charges at a faster rate than it normally would. In this set-up, however, the switching JFET suffers from the same draw-backs as the output FET. That is, the JFET has associated with it a charging capacitance which must be overcome. Further, in order for the JFET to switch the current necessary to quickly charge the gate-to-source capacitance of the output FET, the JFET switch must be biased significantly beyond its threshold turn-on voltage. Relays such as this are also vulnerable to transient propagation between the control and switched circuits.